The first serialises as an array format, containing either a single date in
a double-nested array keyed under the date-parts field, or a date range as
two inner arrays in the date-parts outer array. In this library, array
singles and array ranges are represented separately as Single and Range.
The second form is a field named raw with a string representation of the
date in arbitrary or human formats, which citation software may attempt to
recognise. This library doesn’t attempt to parse raw dates.
EDTF (Extended Date/Time Format) is a structured string format for dates,
datetimes, and ranges established by the United States of America’s Library
of Congress.